An investigator seeks support for five years to examine the role of intra-areal feedback connections in rat visual cortex. Four aims are proposed. The first aim will examine the synaptic organization of the inhibitory circuitry of the feedforward and feedback projections. The second aim will examine how the gain of the thalamocortical pathway might be regulated by feedback projections. The third aim will examine anatomically whether there are direct connections between feedforward and feedback. The final aim will examine whether the feedback projection's effectiveness is modified by use, and if this use dependence operates through an NMDA mechanism.